Self-sealing fuel tanks currently exist in the conventional art. One problem with these conventional self-sealing fuel tanks is that they are manufactured using labor intensive hand lay up processes that require long cure times. Large numbers of self-sealing fuel tanks thus cannot be manufactured over a reasonable time period.
In addition, these conventional manufacturing techniques and materials do not allow for precise control of the outer dimensions of self-sealing fuel tank, a problem where tight fits are required and maximum fuel capacity is desired. A closed molding process using conventional composite construction techniques with conformable elastomeric materials may allow for precise control of the outer dimensions of self-sealing fuel tanks. But there are several challenges that must be addressed.
Air entrapment or air inclusion is a well known problem in the conventional fabrication of fiber-reinforced composites. Air entrapment can result in poor interlaminar adhesion between layers, poor dimensional conformity and less than optimum composite tensile, puncture and impact properties.
One method for making fiber-reinforced composites is the hand layup method. In this method, the fabric intermediates are laid in the impregnation mold by hand and wetted with the matrix. Air is removed from the laminate, by pressing against it with the aid of a roller. This is intended to remove from the layers of fabric not only air present in the laminate structure but also excess matrix material. The procedure is repeated until the desired layer thickness is achieved. Once all the layers have been applied, the component must cure. Curing is performed through a chemical reaction between the matrix material and a curing agent added to the matrix material. The advantage of the hand layup method is the small tool and low equipment outlay. However, this is offset against a low quality of component (low fabric content) and the high level of manual effort, which requires trained laminators.
Hand layup can also be performed as a closed method. The closed method is performed using a vacuum press. Once the fabric mats have been introduced into the impregnation mold, the mold is covered with a release film, a suction fleece and a vacuum film. A vacuum is generated between the vacuum film and the mold. This has the effect of compressing the composite. Any air still included is removed by suction, and the excess matrix material is absorbed by the suction fleece. This means that a higher quality of component can be achieved than with the open hand layup method.
The prepreg method is another closed method. In this, fabric mats which are pre-impregnated with matrix material and have thus already been wetted are laid in the impregnation mold. In this case, the resin is no longer liquid but has a solid, slightly tacky consistency. Air is then removed from the composite by means of a vacuum bag and it is then cured, often in an autoclave, under pressure and heat. Because of the operational equipment required (cooling plant, autoclaves) and the demanding process (temperature management), the prepreg method is one of the most expensive manufacturing methods. However, it also enables one of the highest levels of quality of component.
The vacuum infusion method is another closed method for making fabric-reinforced composites. In this method, the dry fabric layers are laid in an impregnation mold coated with release agent. A release fabric and a distribution medium are placed over this, and this facilitates even flow of the matrix material. A vacuum sealing tape seals the film to the impregnation mold, and the component is then evacuated with the aid of a vacuum pump. The air pressure presses together the parts that have been laid in the mold and fixes them. The suction applied draws the tempered liquid matrix material into the fiber material. Once the fabrics have been completely wetted, the supply of matrix material is stopped and the wetted fiber-reinforced composite material can be cured and removed from the impregnation mold. The advantage of this method is that the fibers are wetted evenly and with almost no air inclusion, and so the components produced are of high quality and there is good reproducibility.
Other techniques of mitigating air inclusion are known to those skilled in the art. In conventional self-sealing fuel tanks bleeder cords are added to the composite structure. These bleeder cords are built into the composite structure and fed out of the mold so as to provide a conduit for air to escape during vacuum molding.
Unfortunately the above conventional composite fabrication methods cannot be used for the manufacture of self sealing fuel tanks that employ a closed molding process. The infusion method cited as one of the most effective methods to prevent air inclusion cannot be used because of the viscosity and the reactive nature of the available elastomeric materials. For instance, a polyurethane reaction mixture retains a suitably low viscosity for an insufficient period of time for the resin to fully impregnate the fabric layers and drive out air. This is because the reaction mixture is primarily a solid at room temperature and unless heat is applied the viscosity will increase as it loses heat. Furthermore, heating the reaction mixture to reduce the viscosity is impractical since the reaction mixture will begin to react when heat is applied and thereby increase in viscosity as the polyurethane polymer is formed. Bleeder cords are problematic since they extend outside of the fuel tank and must be cut even with the volume surface after fabrication of the fuel tank. This generates defects in the volume that must be patched and sealed thereby creating irregular surfaces. Additional defects occur when the cut cords are inadequately patched as frequently happens, resulting in leakage pathways. This approach thus creates an irregular volume surface and is highly dependent upon operator proficiency and skill.
The above conventional methods of composite fabrication are further rendered impractical since the self-sealing fuel tanks contain a self-sealing layer that is impermeable to air. It is not possible for included air that is located inside the self-sealing layer to migrate out of the composite structure since it is trapped between an inner liner and the self-sealing layer, two air impermeable layers. A method to remove trapped air is therefore needed.